When excited hydrogen atoms are driven by collinear static and microwave electric fields, we discover a series of strong-field resonances that can ionize most substates with a given principal quantum number [Formula Presented] We describe a quantal theory using an adiabatic basis that explains the origin of the resonances and why they disappear at certain ratios of the two fields. Our three-dimensional classical Monte Carlo calculations reproduce this behavior. We demonstrate n selectivity sufficient for continued development of this process as a practical detector. © 2000 The American Physical Society.